This invention relates to sensors for use with non-invasive pulse monitors such as plethysmographs or pulse oximeters.
A plethysmograph is a pulse monitor. The plethysmograph sensor shines light into the patient's tissue, and the light transmitted through the tissue is received by a photodetector. The photodetector generates electrical signals corresponding to the transmitted light levels and transmits the signals to a monitor for processing. Arterial blood will absorb some of the light, with more light being absorbed when there is more blood. Thus, changes in the amount of transmitted light are related to pulses of arterial blood in the illuminated tissue.
A pulse oximeter is a device for non-invasively determining the oxygen saturation of arterial blood. The pulse oximeter sensor shines light at two different wavelengths (one in the red range, the other in the infrared range) through a portion of the patient's blood-perfused tissue. The red and infrared light transmitted through the tissue is detected by a photodetector. The amount of light absorbed varies with the amount of oxygen in the blood, and varies differently for red and infrared light. The pulse oximeter monitor computes blood oxygen saturation based on the changes in the two detected light levels between two points in time.
There are several types of sensors for plethysmographs and pulse oximeters. One is a surface sensor in which the light emitter and the photodetector are mounted on the same sensor face. The sensor is attached to the patient with both the light emitter and the detector on the same side of the patient's appendage (e.g., on the patient's forehead). This type of sensor detects light reflected back from the tissue, rather than light transmitted through an appendage. The signal detected will thus be weaker in most cases. The sensor is typically attached with a strap, headband or tape over the sensor, or an adhesive pad between the sensor and the skin.
Another type of sensor is a clamp design, such as that described in U.S. Pat. No. 4,685,464. The durable sensor described in that patent has deformable pads creating conforming tissue contacting surfaces to which the emitters and photodetector are secured. The deformable pads are disposed in a hinged rigid housing that clips on the patient like a clothes pin. This relies on a clamping force to secure the sensor to the patient. The force of the sensor against the patient's tissue could reduce the flow of blood to that region. This exsanguination of the tissue beneath the sensor adversely affects pulse detection and analysis by suppressing the pulse in that portion of the tissue. As a result, the sensor site must typically be checked or moved every four hours to insure adequate perfusion. Because of its relatively large mass, however, the clamp design is more susceptible to signal-distorting motion artifact. i.e., differential motion between the sensor and the patient.
A third sensor design is described in U.S. Pat. No. 4,830,014. The conformable sensor described in that patent has emitters and a photodetector mounted in the same side of a flexible web. The web wraps around a portion of the patient's tissue (such as a finger) so that the light from the emitters must travel through the tissue before reaching the detector. The web attaches to the skin with an adhesive surface on the emitter and detector side of the web. Because of its relatively low mass and the adhesive, this sensor adheres closely to the patient's skin and minimizes the effects of motion artifact. In addition, its flexibility and use of adhesive to secure it minimizes the exsanguination caused by rigid sensors. Thus the sensor site typically only needs to be checked every eight hours.
One problem with such a conformable sensor is that of electromagnetic and photic interference with the detector. In the clamp-type of sensor, a Faraday shield, which is a grid of wires, covers the emitter to block out electromagnetic interference. In addition, shielded cabling is used to prevent interference with the conductive lines carrying the signal from the detector. The use of such shielded cabling to connect up to the detector on a conformable sensor makes the sensor bulky and more expensive.
In one type of sensor, disclosed in U.S. Pat. No. 4,621,643, a coding resistor is attached to the probe to indicate the wavelength of light emitted by the particular emitter. The selecting and attaching of such a coding resistor to a conformable sensor makes the manufacturing process difficult and expensive.
It is desirable to have an inexpensive conformable sensor which is easy to manufacture and provides appropriate shielding and a coding resistor.